mdadm/tests: names_template enhance

mdadm/tests: names_template enhance

For super1, if the length of hostname is >= 32, it doesn't add hostname
in metadata name. Fix this problem by checking the length of hostname.
Because other cases may use need to check this, so do the check in
do_setup.

And this patch adds a check if link /dev/md/name exists.

Signed-off-by: Xiao Ni <xni@redhat.com>
Signed-off-by: Mariusz Tkaczyk <mariusz.tkaczyk@linux.intel.com>